Conventionally, various types of apparatuses have been provided for cooling an extruded thin film by bringing it into direct contact with coolant in the process of making a tubular thin film from thermoplastic resin.
For example, the following are known: quenched solidification method through direct pressure against and retention of overflow liquid flowing down on the outer wall of internal overflow pipe (Japanese Examined Patent Publication No. 35192/1970); the method of improving accelerated film production performance by providing a helical channel through which coolant flows in the lower part of double tube for pipe size control through which overflow liquid flows down (Japanese Examined Patent Publication No. 31473/1971); the method of cooling and solidifying through contact with coolant and of removing by absorbing adhesive liquid (Japanese Examined Patent Publication No. 2072/1964); and an internal mandrel for a cooling tubular thin film that is provided with more than one stage of an upper annular slit nozzle for jetting coolant (Japanese Unexamined Patent Publication No. 47795/1994).
The most important technical point for producing thin film that has excellent extensibility and long-term stability lies in how to manufacture substantially formless thin film in continuous and stable manner. This is important, in particular, in film production equipment of tubular method that uses an internal cooling mandrel, and determines efficiency of the entire equipment and production quality of a tubular thin film and of a sheet.
The inventors precedently proposed the multi-stage cooling that could enhance cooling ability and significantly improve accelerated film production (Japanese Unexamined Patent Publication No. 47795/1994). Although this enabled substantially accelerated production of a tubular molten thin film, it has a disadvantage that further acceleration will increase air entrained with the thin film or coolant, thus leading to increased air quantity contained in the coolant, which results in accumulation of air space in the lower part of the pipe size control ring.
This air space adversely affects moldability of molten thin film. More specifically, although molten thin film achieves a balance by head pressure of an external water tank and hydraulic pressure of internal coolant, when the said air space gets off balance and the balance disproportionates, the air space passes between the thin film and the tubular control ring, reaches an annular nozzle for jetting coolant in the upper part, bursts in an area where the molten thin film enters the coolant and causes cooling irregularity of the thin film or pores.
These problems have considerable effect on the entire equipment, such as reduced runnability due to inability of continuous drawing, and deteriorated quality of the tubular thin film or sheet due to cooling irregularity.
In the following, we describe a conventional cooling mandrel in detail with reference to the attached drawings.
FIG. 4 is a detailed illustration of a cooling unit of the conventional cooling mandrel.
Conventionally, an inner surface of an tubular thin film 4 is cooled by internal coolant and coolant from an external cooling tank 22 that are uniformly distributed by a helical channel 9 provided in the lower cooling unit 10.
The tubular thin film then cooled down achieves a balance by internal pressure of the lower coolant and head pressure of the external cooling tank, and enters a seal ring unit 15 without contacting the helical channel 9 of the lower cooling unit 10, or without separating from the bottom of a pipe size control ring unit 6.
However, as film production is accelerated, air entrained with the molten tubular thin film 4 or contained in the coolant from an upper annular nozzle for jetting coolant 8, a middle annular nozzle for jetting coolant 7 and the lower cooling unit 10 also increases.
During film production the air stays as air space 12 in the bottom of the tubular control ring unit 6. Although the coolant running through the helical channel 9 in the lower cooling unit 10 is drained from a lower exhaust port 14, the air space 12 residing in the bottom of the tubular control ring unit 6 expands as film production time passes, rather than being vented to the lower exhaust port 14.
In the internal cooling mandrel, the air space 12 destroys the balance between the internal pressure of the lower coolant and the head pressure of the external cooling tank 22 in the lower cooling unit 10, and rises as bubble to the middle annular nozzle for jetting coolant 7 or the upper annular nozzle for jetting coolant 8, through a gap between the tubular thin film and the pipe size control ring unit 6. Then, as the air space 12 rising as bubble bursts in the vicinity of the upper or middle annular nozzle unit for jetting coolant, the molten tubular thin film 4 suffers from uneven thickness due to irregular cooling or generation of pores.